In the art of magnetic resonance imaging (MRI) it is known to use switched-mode power converters to power gradient coils of MR examination systems. Switched-mode power converters use pulse width-modulation (PWM) at a fixed effective fundamental switching frequency, for instance in the range of 20-100 kHz. Higher harmonics of the fixed effective fundamental switching frequency might interfere with MR measurement signals within a measurement signal bandwidth of MR imaging and/or spectroscopy, for either protons (hydrogen nuclei) or other nuclei, thereby deteriorating an MR measurement signal quality. The switched-mode power converter commonly is equipped with an output filter for each gradient coil axis to attain a sufficient attenuation of the higher harmonics.
In multi-species nuclei MR scanning, an MR measurement signal frequency can be as low as 4.6 MHz, e.g. for 14N at a magnetic field strength of the main magnet of the MR examination system of 1.5 T. In cases like this it will become difficult to attain sufficient attenuation of the higher harmonics. Too much filtering might result in too much loss of a gradient coil power amplitude. This is a disadvantage since the purpose of the power supply unit is to deliver as much power to the gradient coil(s) as possible. As another drawback, the power loss results in power dissipation within the filter which must therefore be cooled, so that additional hardware becomes necessary, incurring additional costs.
The problem is especially pronounced in multi-species nuclei MR measurements, in which the MR measurement signal frequencies lie within an MR measurement signal frequency range of relatively narrow bandwidth of, for instance, a few hundred Hz. An unavoidable temporal drift of the magnetic field strength of the main magnet will change the MR measurement signal frequencies, so that one of the harmonics of the fundamental switching frequency might fall into the frequency MR measurement signal frequency range.
It is therefore desirable to avoid this type of interference in MR examinations systems.